N-Methylnaltrexone bromide (CAS No. 73232-52-7) is a known pharmaceutically active compound which is used in particular for the treatment of post-operative ileus or for the treatment of opioid-induced constipation. N-Methyl-naltrexone bromide is also referred to as naltrexone methobromide and corresponds to the chemical formula:

The compound N-methylnaltrexone bromide is described in U.S. Pat. No. 4,176,186. The compound is prepared, for example, by reacting the free naltrexone base with methyl bromide. This process has the disadvantage that the reaction must be carried out in a pressurised vessel in an organic solvent, such as anhydrous acetone and/or dimethylformamide, for a comparatively long period of time. In addition, undesirable secondary products are formed in the reaction. In U.S. Pat. No. 4,176,186 it is also proposed to methylate the free naltrexone base with methyl iodide or dimethyl sulfate in a first step and then exchange the anion present in the resulting compound for the bromide anion by means of chromatography. A bromide-loaded chromatographic ion exchanger is used thereby. Chromatographic ion exchange is not suitable for the preparation of methylnaltrexone bromide in relatively large amounts, however, because it is very intensive in terms of time, volume and solvent and is therefore too expensive for industrial use. There is therefore a need for a process for the preparation of methylnaltrexone bromide that can be carried out simply and inexpensively on an industrial scale.